In the bulk or solution polymerization of one or more monomers, it is generally necessary to remove unreacted monomer, dimers, trimers, oligomers, and diluent if present from the resulting polymer. There are in theory and practice a number of processes for reducing the residual content of volatile material in the polymer melt.
In the technique to which the present invention relates, a polymer melt from the reactor is pumped by a gear pump or other suitable means to a heater (sometimes called a preheater) on top of a vertical vacuum chamber (i.e. devolatilizer). The heater is typically a shell and tube type heat exchanger. The polymer melt leaving the heater may be forced through many fine holes of a distributor showerhead. Typically the hole diameter ranges from 1/32 of an inch to 1/8 of an inch. The strands of polymer melt which are formed descend towards the bottom of the vacuum chamber (hence, the name, a falling strand devolatilizer). The showerhead extrudes the polymer melt as fine strands to lower the diffusion path distance. (The characteristic time for diffusion is defined by the equation .lambda..sub.D =.sup.r2 /D where .lambda..sub.D is the diffusion time, r is the radius of the strand and D is the diffusion coefficient.) The residual monomer and, if present, diluent, and dimers and trimers devolatilize out of the descending or falling polymer strands as they are exposed to the vacuum (and it is maintained) causing the polymer to foam. The polymer melt which collects at the base of the devolatilizer is then forwarded to the stranding and pelletization unit operations.
The preheater, showerhead distributor, and devolatilizer are maintained at an elevated temperature to reduce the viscosity of the polymer melt. The increase in temperature also increases the vapor pressure which increases the mole fraction of volatiles in the vapor phase (i.e. y.sub.i P=.delta..sub.i x.sub.i P.sub.i.sup.vap wherein y.sub.i is the mole fraction of the volatile component in the vapour phase; P is the system pressure; .delta..sub.i is the activity coefficient of the volatile component; x.sub.i is the mole fraction of the volatile component in the melt phase; and P.sub.i.sup.vap is the vapour pressure of the volatile component). This permits the bubbles of volatile material to diffuse or rise to the surface of the strands of polymer melt more rapidly. However, reducing the viscosity of the polymer melt increases the rate at which the polymer flows in the form of a strand to the bottom of the devolatilizer and correspondingly reduces the residence time of the polymer melt in the vacuum chamber. There is a need for a method to increase the residence time of the polymer melt in the falling strand vacuum chamber devolatilizer.
There are a number of devices which may be used to increase the residence time of a polymer melt in a vacuum chamber devolatilizer. Representative of such art are U.S. Pat. No. 3,694,535 issued Sep. 26, 1972 which teaches a single annular distributor device; U.S. Pat. No. 4,934,433 issued Jun. 19, 1990 which teaches a single "serpentine" distributor which replaces the conventional shell and tube heat exchanger in a falling strand devolatilizer, U.S. Pat. No. 5,118,388 issued Jun. 2, 1992 which discloses a single "candy scoop" distributor for a falling strand devolatilizer; and U.S. Pat. No. 5,069,750 issued Dec. 3, 1991 which teaches a single distributor tray for use in a falling strand devolatilizer. None of the above references teach an array of trays nor do they teach the type of tray which may be used in accordance with the present invention.
U.S. Pat. No. 3,747,304 issued Jul. 24, 1973 discloses a foam reduction means for treating hydrocarbon streams (typically oil or partially refined oil) in a separator drum to remove entrained vapor such as air. The separator drum is operated under pressure, rather than vacuum. While the drum contains an array of trays, the trays are designed to permit the liquid to flow from the tray and the foam to be retained upon the tray. The whole objective of U.S. Pat. No. 3,747,304 is to permit the liquid portion of the stream to flow as rapidly through the drum as possible while retaining the foam on the trays. The trays in the array according to the present invention do not contain an under flow weir to retain the foam and permit the non-foamed polymer to flow through the devolatilizer. The whole intent and essential feature of the design of the 304 patent teaches away from the subject matter of the present invention.
U.S. Pat. No. 3,886,049 issued May 27, 1975 is most intriguing. The patent discloses and claims a process for the recovery of aromatic monomer from polymers. In the process, a falling strand devolatilizer is used. However, there are no distributor trays in the vacuum chamber. Rather two vacuum chambers are used in series. More importantly there is a separator, downstream from the devolatilizer, to separate dimers and trimers from the aromatic monomer. The patent is interesting in that the patentee was likely aware of the art of the 304 patent but did not consider it useful in a devolatilizer and only used the art in the separation of the monomer from the heavy dimer/trimer organic phase. Clearly, although the art was available relating to the use of offset weirs in the refining and separation arts, the patentee of the 049 patent did not consider this art to be relevant to the devolatilization art field and particularly to the use of distributor trays in a falling strand devolatilizer.
The present invention seeks to provide a polymer devolatilizer tray and an array of devolatilizer trays which may be used in falling strand devolatilizers to improve the efficiency of the removal of monomer and diluent if present